A conventional fire alarm system will provide an adequate scheme for the protection of many small buildings. In larger, more complex buildings however, more sophisticated, addressable, fire alarm systems tend to be used. These systems offer benefits in speed of detection, identification of the location of a fire and easier maintenance. Such systems also provide tolerance to faults in the wiring. This in turn enables a single pair of wires to be used to connect 100 or more devices to the system. This results in cost savings in the cabling and installation. In larger installations, the benefits of improved maintenance and reduced cabling cost are considerable.
One such system has been disclosed and claimed in U.S. Pat. No. 4,916,432 which issued Apr. 10, 1990 and is entitled Smoke and Fire Detection System Communication. The '432 patent is assigned to the assignee hereof and is incorporated by reference herein.
FIG. 1 illustrates an example of a single loop addressable system 10 of a known type. Loop wiring 12 connects to the control unit 14 at each end. Detectors 20a, call points 20b, sounders 20c and interface modules 20d are wired directly to the loop, each having its own address.
The control unit 14 communicates with each device on the loop. If an alarm or fault condition is signaled, or if communications are lost with one or more detectors, the appropriate response is triggered. The loop 12 can be powered from each end so that if it is broken at any point, none of the devices are disconnected from the control unit 14. In addition, the use of short circuit isolators 20e can minimize the area of coverage lost in the case of a short circuit.
Known addressable systems use the same pair of wires to supply power to the loop, and to communicate with devices on the loop. The communication method, or protocol used varies from manufacturer to manufacturer. Known protocols usually switch the supply voltage (typically 24V) to other voltage levels to achieve communication.
One protocol is disclosed in the above noted '432 patent. It will be understood that the above is exemplary only. The present invention can be used with a wide variety of protocols without limitation.
A typical basic protocol has two main parts, a query or poll of a device by the control panel including the device address and control information, and a response from the device giving its status and other information. Precise details of the information transferred will depend on the protocol. In a known form of processing, each device on the loop will be polled in turn. To increase speed around a loop, some protocols allow polling of groups of devices on a single communication.
Known units, such as smoke detectors and the like, all without limitation are usually assigned an unchangeable serial number during manufacture. A unit's, or detector's address is a settable, indicium apart from the unit's serial number.
Different addressable systems provide a variety of structures and methods of setting the address of a device. These include without limitation manually settable switches, dedicated addresses or address cards and automatic, according to the position of the device on the loop.
As described above, addressable monitoring systems, fire, intrusion gas or the like, are based upon a set of devices. The devices are called in some order by a system control unit, for example a fire alarm control panel. Each device is identified by its own address. Typically the control unit, or panel, sends a message containing the address of each installed device sequentially. If the device receives a message with its address, it responds to the panel, sending back the requested information, for example, by sinking current. Otherwise the device ignores the message.
In an addressable monitoring system, such as a fire system, if two devices have the same address, proper communication will not be possible. This ‘double address’ is seen as a fault by the control panel because it is related to a current sink double that which is normal.
Designing, installing and trouble shooting such monitoring systems can be a difficult task which increases in complexity the bigger the system is. Wrongly setting device addresses is a common occurrence at commissioning time. Such errors can be particularly problematic in large installations where investigation of a large number of devices is required to identify the offending unit or units.
To help with solving the problem, some control units, or panels, employ double address detection circuitry. Whilst this may improve the situation, such circuitry can usually detect abnormal current loading which may, in fact, be caused by problems other than double addressing. This in itself can result in wasted time spent looking for double address faults which don't exist.
Performing double address detection in the control unit also limits the available loop length because double current capability above normal operation must be available for detection. Hence using a method of double address discrimination that does not rely on sensing a double current load could increase the potential loop length substantially.
There is thus a continuing need to be able to detect units that have erroneously been assigned a common address. Preferably solutions to this problem will not require modifications of or additions to the detectors or units to be installed in such monitoring systems. It would also be desirable to be able to upgrade existing monitoring systems with such capabilities.